JPS6411885B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonically actuated device for measuring the flow velocity of a medium, which device comprises an ultrasonic transmitter and receiver defining a propagation path running along the axis of a measuring tube. Equipped with.

In known devices of this kind, the measuring tube is passed between two bent tubes, with the transmitter and the receiver being arranged in each case in the outer wall of the bent tube. However, there are also many devices in which the transmitter and receiver are offset in the axial direction and are arranged on mutually opposite sides of the measuring tube wall. Furthermore, a device is known in which the measuring tube is made of plastic and the transmitter and receiver are installed in the measuring tube in such a way that a zigzag-shaped propagation path is obtained in which the ultrasonic waves are reflected many times on the measuring tube wall. It is.

During operation, the transmitter is excited by an impulse, so that the transmitter emits an ultrasonic signal in a short time. The propagation time of this ultrasonic signal until it reaches the receiver is measured. If the propagation time in the flow direction as well as the propagation time in the direction opposite to the flow direction is measured in a flowing medium, it is possible, for example, to determine the flow velocity of the medium or its density. In this case, accurate measurement of the propagation time of the ultrasonic signal is important. This presupposes that it is possible to precisely determine the point in time when the wavefront reaches the receiver.

It has been shown that as the size of the measuring tube and thus the diameter decreases in the case of a circular cross section, the propagation time measurement becomes less accurate. However, measuring tubes with small cross sections are highly desirable, for example if relatively small volumes of medium with relatively high flow rates are to be conducted.

The object of the invention is therefore a device of the type mentioned at the outset, which has a very small tube cross-section and which nevertheless allows accurate propagation time measurements.

According to the invention, the measuring tube 1 is made of metal and the inner wall coating 2 is made of plastic, which has a smaller acoustic impedance than metal.
, and this inner wall coating 2 has an inner diameter di smaller than about 15 times the wavelength of the ultrasound signal.

This structure allows accurate propagation time measurement in measurement tubes with small cross-sectional areas because a considerable amount of energy is applied to the receiver by the preceding waves before the wavefront formed by the fundamental wave arrives. It is based on the startling realization that this will fail. These leading waves are waves of the first and second types emitted by the crystal of the transmitter, and according to the wave equation, it was recognized that they propagate through the medium at a higher speed than the plane fundamental wave. The present invention takes advantage of the fact that the first type and second type waves have propagation directions that form an angle with the propagation direction of the fundamental wave. In other words, when the fundamental wave is radiated in the axial direction of the measuring tube, the fundamental wave reaches the receiver without attenuation, but the first and second type waves hit the tube wall and are reflected there. It is attenuated due to the small acoustic impedance of the inner walls. The leading wave is therefore so weak that it is ensured that the receiver is not excited by it.

The speed of sound in most liquids is between 1500 and 1800 m/s. For water it is 1500m/s. The superaudible frequency should be chosen as high as possible, so that good resolution is obtained. However, too high a frequency results in large transmission losses. A good value for superaudible frequency is 1MHz. At this frequency, the inner diameter of the measuring tube used to measure water is 20-25 mm. Measurements have shown that there is no problem with the leading wave in metal measuring tubes with a diameter of 25 mm or more, but problems occur when the diameter is less than 20 mm. The same applies for other liquids and other superaudible frequencies.

The length of the propagation path must not fall below a certain minimum value in the device considered here, since if the distance between the transformers is too small, the time difference will be too small for acoustic measurements and the This is because measurement results cannot be obtained. This lower limit is purely empirical and depends on the equipment and measurement technical aspects. Usually at least 40-50 times the wavelength of the ultrasound signal.
However, if the ultrasonic transmitter crystal has a smaller cross-sectional area than the measuring tube cross-section and this tube cross-section has to be completely filled with sound waves in order to obtain the average value, then for example 150-200 of the wavelength A propagation path twice as long is often used. However, propagation paths that are 300 to 500 times the wavelength may also be used. In both cases, the reflection with attenuation of waves of the first and second types, which is the object of the invention, is obtained. Reflections often cause correspondingly strong attenuation.

In particular, the inner wall of the measuring tube can consist of plastic, preferably polyamide. Plastics inherently have a significantly lower acoustic impedance than metals, or can be easily finished to have such an impedance.

It is sufficient simply that the inner wall coating consists of plastic. The measuring tube itself may therefore be made of metal. This is often recommended when high requirements are placed on the strength or temperature stability of the measuring tube.

It has been found that a small wall coating thickness is sufficient to achieve the desired results. The thickness is
It should be between 0.5 and 1.5 mm, advantageously about 1 mm.

The present invention will now be described in detail with reference to embodiments shown in the accompanying drawings. The figure represents a device according to the invention in a diagrammatic and partly sectional view.

The measuring tube 1 is provided with an inner wall coating 2 made of polyamide. The measuring tube has at one end a head 3 which houses the supply tube 4 and, in an axial extension of the measuring tube 1, an ultrasonic transformer 5. The other end is provided with a head 6 which houses the discharge tube 7 and, in an axial extension of the measuring tube 1, a second ultrasonic transformer 8.

The two transformers 5 and 8 form a control and measuring circuit 9
Connect with. The circuit first sends an excitation impulse to the transformer 5, which then acts as a transmitter to generate an ultrasound signal. The ultrasound signal received by the transformer 8 as a receiver is transmitted to the circuit 9 so that the circuit can determine the propagation time of the ultrasound signal. Since the medium is fed in the direction of arrow P, this propagation time measurement is carried out in the flow direction. In a second step, the control and measuring circuit 9 sends the excitation impulses to the transformer 8, which acts as a transmitter for the next ultrasonic signal. The ultrasonic signal received by the transformer 5, which now acts as a receiver, is transmitted to a circuit 9, in which case the propagation time in the opposite direction to the flow direction is measured. Based on these measurement results, the arithmetic circuit 10 connected to the circuit 9 can determine and indicate the flow rate since the flow velocity and pipe cross-sectional area are given in advance. From the measurements, the speed of sound in the medium and thus also the density of the medium can be determined.

In the embodiments described above, the medium has a sound velocity of approximately
It is water with a velocity of 1500 m/s. The ultrasonic transformer operates at a fundamental frequency of 1MHz. The wavelength of ultrasonic vibration is
It is 1.5mm. The measuring tube has an outer diameter of Da20mm and a wall thickness of 1
mm, thus obtaining an intermediate diameter Dz18 mm.

Similarly, plastic coating 2 has a film thickness of 1
mm, the inner diameter Di16mm can be obtained. This corresponds to approximately 11 times the wavelength. The length L of the propagation path is approximately 50
cm, which corresponds to 333 times the wavelength. In this structure, the waves preceding the original wave front are so small that they can be ignored. In contrast, when using measuring tubes made of steel with the same internal diameter, a very significant leading wave is obtained.

Similar good results have been achieved with plastic coatings.
It was also achieved using 0.5 and 2.0 mm. The same goes for small internal diameters, e.g. 10 or 14mm, as well as large internal diameters, e.g. 20mm and other different lengths L, e.g. 20
This also applies to cm or 75cm.

[Brief explanation of drawings]

The accompanying drawings show schematically and partly in section an embodiment of the device according to the invention. 1... Measuring tube, 2... Inner wall coating, 5...
. . . ultrasonic transformer, 8 . . . ultrasonic transformer, 9 . . . control and measurement circuit.

Claims (1)

[Claims] 1. A device for measuring the flow velocity of a medium, operating with ultrasound, comprising an ultrasonic transmitter and a receiver for determining a propagation path running along the axis of the measuring tube 1. is made of metal and has an inner wall coating 2 made of plastic having an acoustic impedance smaller than that of metal, and characterized in that the inner wall coating 2 has an inner diameter di smaller than about 15 times the wavelength of the ultrasound signal. device to do. 2. The device according to claim 1, wherein the inner wall coating 2 has a thickness of 0.5 to 1.5 mm. 3. The device according to claim 1 or 2, wherein the plastic is polyamide.